Research has been conducted in recent years for reducing levels of reflection return light of liquid crystal screens of televisions and cell phones by providing an anti-reflective structure having a fine relief structure in which the period of the relief structure is controlled to be equal to or less than the wavelength of visible light. As an example of a method used for this purpose, a fine relief structure is formed by treating aluminum with alumite and the relief portion is transferred to a molding material such as a resin to produce an anti-reflective article. The relief pattern formed by alumite treatment has been reported to have a pyramid form such as that of cones or rectangular cones.
In the case of treating aluminum with alumite and producing a transfer by using the surface as a casting (stamper), since the surface of the stamper is directly reflected on the transfer, the quality of the stamper surface is important. Examples of factors that affect this surface quality include second phase particles, crystal grain size and crystal orientation in the material, and mirrored surface polishability of the material. There are considered to be offsetting aspects between the second phase particles and other parameters (crystal grain size, crystal orientation and mirrored surface polishability).
In general, defects occur during alumite treatment when second phase particles having an equivalent diameter equal to or greater than roughly the wavelength of visible light are present on the surface. These defects are manifest as surface irregularities of the stamper transfer, and are undesirable since they impair the absorption or scattering of reflected light. On the other hand, even in the case second phase particles smaller than the wavelength of visible light are present on the surface, they can also cause defects during alumite treatment. If these defects are present continuously or are intermittently present at an interval equal to or less than roughly the wavelength of visible light, the same problems as described above occur, thereby making this undesirable. In addition, in the case second phase particles have an equivalent diameter equal to or greater than the wavelength of visible light and are scattered on the surface at a short distance equal to or greater than the equivalent diameter, defects similarly occur during alumite treatment. However, disturbances in absorption or scattering of reflected light attributable thereto are not recognizable to the naked eye.
Consequently, second phase particles that can cause defects in alumite-treated films are preferably present in aluminum at as low a level as possible. The use of pure aluminum that contains low levels of added elements and impurities capable of forming these second phase particles has been determined to allow the obtaining of a fine relief structure free of treated film defects. Consequently, pure aluminum has come to be used as a stamper material (see, for example, paragraph [0025] of Patent Document 1).
In addition, Patent Document 2 describes to the effect that anodized porous alumina having superior regularity can be produced by forming an aluminum alloy film containing 0.5% by weight to 10% by weight of Mg on an aluminum substrate, and then carrying out alumite treatment on the aluminum alloy film.